Some cars were parked in the school supply store parking lot. 2 more cars park at the school supply store. Now there are 20 cars in the parking lot. How many cars were parked at the school supply store at the beginning?
The number of cars parked at the store at the beginning is the difference of the number parked now and the number that came and parked earlier. The difference is $20 - 2$ cars. $20 - 2 = 18$ cars.